1. Field of the Invention
The present invention relates generally to a method and system for transmitting data in a wireless communication system, and in particular, to a method and system for transmitting data using a band Adaptive Modulation and Coding (AMC) scheme in an Orthogonal Frequency Division Multiple Access (OFDMA) system.
2. Description of the Related Art
A cellular system is a typical type of the mobile communication system. A “cellular system” is a system in which a base station (BS) communicates with a subscribed station (SS) through a wireless channel and the BS is connected to a wired network. A cellular mobile communication system using a Code Division Multiple Access (CDMA) scheme (hereinafter referred to as a “CDMA cellular mobile communication system”) is a typical system of the cellular system.
Although the cellular system was fundamentally developed to support voice communication, various systems capable of providing a data service have been recently proposed. In addition, the amount of data required by each user is steadily increasing, as users demand to transmit larger amounts of data at higher speeds. Therefore, various research is being conducted for a CDMA cellular system to meet these demands.
Additionally, research into an Orthogonal Frequency Division Multiplexing (OFDM) system, which is different from the CDMA system, is being made to provide users with the larger amount of data at high speed. The commercialization of the OFDM system is now under discussion. For example, the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standardization group, one of the international standardization groups, is about to establish an IEEE 802.16d standard as a standard for providing a broadband wireless Internet service to a fixed SS.
The OFDM scheme can be defined as a 2-dimensional access scheme, which is a combination of a Time Division Access (TDA) technique and a Frequency Division Access (FDA) technique. IEEE P802.16d/D3-2004 uses an Orthogonal Frequency Division Multiple Access (OFDMA) scheme which divides a full frequency band per subcarrier, groups the subcarriers into subchannels each having a predetermined number of subcarriers, and separately allocates the subchannels to users. Thus, all SSs in the system use subchannels, each comprised of subcarriers dispersed over the full frequency band. Therefore, a system using the OFDMA scheme (hereinafter referred to as an “OFDMA system”) carries data on some of the subcarriers included in a particular subchannel.
In a broadband OFDMA system, all SSs connected to a BS share a channel, and an interval for which each SS uses a channel is allocated by the BS every frame. Therefore, the BS divides access information into uplink access information and downlink access information, and arranges the uplink access information and the downlink access information at the head of each frame, before transmission to all SSs.
FIG. 1 is a diagram illustrating a frame structure including an uplink and a downlink in a conventional broadband OFDMA system. In FIG. 1, a vertical axis represents subchannel numbers 147 (S, S+1, S+2, . . . , S+L) and a horizontal axis, which is a time axis ‘t’, represents OFDMA symbol numbers 145.
Referring to FIG. 1, a frame is time-divided into a downlink (DL) 149 and an uplink (UL) 153, and an OFDMA frame includes a plurality of OFDMA symbols, for example, 12. An OFDMA symbol includes a plurality of subchannels, for example, L. Such an OFDMA system is a communication system that acquires a frequency diversity gain by dispersing data subcarriers used therein over the full frequency band.
The downlink 149 includes at the head thereof, a preamble 111 for acquiring synchronization between a transmitting SS and a receiving BS. The downlink 149 also includes broadcast data information such as a frame control header (FCH) 113, a DL_MAP 115, and a UL_MAP 117, and finally includes DL bursts 121, 123, 125, 127, and 129 for transmission symbols.
The uplink 153 includes at the head thereof, preambles 131, 133, and 135 for acquiring synchronization between a transmitting SS and a receiving BS. The uplink 153 also includes UL bursts 137, 139, and 141, and includes a ranging subchannel 143 for controlling reception power of the BS. Information on positions and allocation of the UL bursts 137, 139, and 141, and the DL bursts 121, 123, 125, 127, and 129 are provided to the SS by the BS through the DL_MAP 115 and the UL_MAP 117, and based on this information, the SS is variably allocated a subchannel, which is a combination of a frequency and a symbol, every frame, and communicates with the BS using the allocated subchannel. That is, the SS uses a different subchannel every frame, instead of a fixed subchannel.
The frame has a predetermined time gap, i.e., a Transmit/receive Transition Gap (TTG) 151 for which no data transmission is achieved, in an interval where transition from a downlink to an uplink occurs. In addition, the frame has a predetermined time gap, i.e., a Receive/transmit Transition Gap (RTG) 155 for which no data transmission is achieved, in an interval where transition from an uplink to a downlink occurs, i.e., where transitions from one frame to another frame occurs. After a lapse of the transition time, a preamble region is provided for an SS to acquire system synchronization.
In this frame, data for each SS is coded and modulated according to the best coding rate and modulation scheme before being transmitted through a wireless channel. For example, the coding rate and the modulation scheme can be selected according to a position of an SS located in a cell. That is, a change in state of a channel between a BS and an SS depends upon a position of the SS.
As to the channel state depending on the SS's position, an SS located in the center of a cell tends to have the best channel state. Therefore, the SS located in the center of a cell uses a high-efficiency modulation scheme, i.e., a high-order modulation scheme, and a high-transmission efficiency coding rate, i.e., a high coding rate. However, an SS located in a cell boundary has the worst channel state due to interferences from neighbor BSs. Therefore, the SS located in the cell boundary uses a low-efficiency modulation scheme, i.e., a low-order modulation scheme, and a low-transmission efficiency coding rate, i.e., a low coding rate, thereby securing safe data transmission.
However, in the conventional OFDMA system in which a subchannel including subcarriers dispersed over the full frequency band is used, high-efficiency modulation scheme and coding rate can be used only when it is determined that a state of a channel through which data is to be transmitted is good. Therefore, it is impossible for an SS to change a coding rate and a modulation scheme according to its channel quality using only some frequency bands, quality of which is determined to be good.
Generally, even the channel, a state of a subchannel of which is reported to be very bad, can show high quality because a subchannel includes a plurality of frequency bands, i.e., a plurality of subcarriers. That is, among subcarriers included in a particular subchannel, a state of which is reported to be bad, particular subcarriers can show high quality. In this case, therefore, data transmission efficiency can be increased if a method for transmitting data with high-efficiency modulation scheme and coding rate using only the frequency bands showing high quality is used instead of a method for selecting a modulation scheme and a coding rate depending on qualities of all channels.
When an SS is located in a cell boundary, its channel quality often deteriorates due to interference from other cells. If a particular frequency band in which an SS can have good channel quality even in the cell boundary is provided, the foregoing efficiency transmission can be achieved using this band. A channel previously reversed in each cell for this purpose is called a “safety channel.” The safety channel is well disclosed in Korean patent application No. 2004-15218 filled by the applicant on Mar. 5, 2004. However, there is no currently available method for providing UL_MAP (uplink mobile application part) and DL_MAP (downlink mobile application part) for the safety channel.